Values for vascular plant species density as a principal feature of diversity are calculated for 57 islands and archipelagos worldwide and for 30 Macaronesian Islands. Correlation analysis confirms the significance of certain ecological and biogeographical factors for the number and composition of the species. It can also be shown that neither the distance from the mainland nor the proportion of endemics - both of which have been used in the past as indicators of the degree of isolation - are significantly correlated with the α-index which indicates the extent of the species diversity. Various hypotheses which are based on the richness or paucity of endemic species are discussed.